High Performance, Economical Substrates: Myth or Reality - Taconic

High Performance, Economical Substrates: Myth or Reality?
By Nick Santhanam & revised by Jim Francey
Taconic Advanced Dielectric Division
Introduction:
In the past decade, the wireless industry has undergone a metamorphosis. This industry has turned from a
predominantly military driven market to a cost conscious, consumer driven commercial market. At the same
time, the wireless applications are moving up in the frequency spectrum. A typical example would be the PCS
system. The PCS systems are currently designed for 1.9 GHz spectrum, but the newer design for wireless
communications are moving up to 5.8 GHz. As the commercial applications move up in the frequency
spectrum, there is a renewed interest in high performance, high frequency low cost substrates. In the various
military applications, PTFE substrates were always the materials of choice. With a very low loss tangent
(0.0018 at 10 GHz), excellent resistance to processing chemicals, negligible water absorption and a high
temperature resistance (PTFE, popularly known as Teflon®, has a melting point of 621 o F [327 o C]), PTFE
substrates were able to meet all the technical requirements of RF/wireless design. When commercial
applications demanded high performance substrates, there were certain disadvantages with conventional
PTFE substrates. Firstly, PTFE substrates were expensive; typically the price ranged from 8-10x the price of
the epoxy glass laminates (popularly called FR-4). Secondly, due to the inert nature of PTFE, the processing
of the PTFE substrates needed one extra step viz. pre-treating the hole wall by a plasma process or by a
chemical process (such as a sodium napthanate etch) to achieve excellent adhesion of the copper to the hole
wall. Thirdly, conventional PTFE substrates were “soft”, meaning that they needed special care during the
processing step. Finally, the CTE of conventional PTFE substrates were high (typically, 180-250 ppm/ o C
depending on the dielectric constant of the substrates).
To fill the requirements in the commercial arena, several proprietary and generic products were attempted.
These products did offer some advantages over the conventional FR-4 substrates, but all of these products
were “me too” products, and did not match or exceed the superior electrical performance of PTFE nor did they
offer the cost advantages of FR-4.
Why do these products not meet the requirements? Most of these replacement products are thermoset resin
(thermoset resins are resins which form an non-meltable mass after cross-linking) based system. Due to the
chemical nature of most of the thermoset systems, they burn when exposed to a open flame. To make these
resin systems viable for PCB use, flame retardant additives (a bromine or a chlorine based compound) has to
be added. Hence, even though the electrical performance of the pure resin might be able fill the need at a
high frequency, the addition of a flame retardant to render the resin useable for a PCB, renders the PCB
unusable for a high frequency use. For example, polybutadiene has a loss tangent of 0.0005 at 10 GHz, but
the loss tangent (tan δ) increases to 0.006 with the addition of a flame retardant additive and other fillers (a
12x increase). Furthermore, as some European countries (for e.g. the Scandinavian countries) become
extremely sensitive to the effect of their products/process on the environment, the thermoset resin based
systems, which are processed in an organic solvent medium, might not be an environmentally friendly
alternative. Most of the thermoset-based systems have an unsaturated, exposed carbon atom, hence leading
to high moisture absorption and to a greater tendency to absorb processing chemicals during the processing
stage. This leads to two unwanted developments:

1. The tendency to adsorb processing chemicals during the processing creates the need to add an extra
step of baking after the drilling step (hence the advantage over PTFE substrates of eliminating the
process of hole treatment is lost) to eliminate the blistering during the solder reflow process.
2. A high absorption leads to a shift in dielectric constant (hence a shift in resonant frequency), a higher
loss tangent, and a phase shift with frequency. This is an important factor in applications, where the
PCB is exposed to high humidity, such as an antenna in a base station application.
From an economic standpoint, these products are cheaper than conventional PTFE substrates, but they are
definitely much more expensive than FR-4. Furthermore, a typical thermoset resin has a very low viscosity
past its melt temperature. Hence when the laminates are manufactured under high temperature and pressure,
the resin tends to flow. This phenomenon tends to give a “pillow effect”, which is a greater variation of
dielectric constant and thickness within a sheet, than most RF designs can handle. For example, a greater
variation of thickness within a sheet can adversely affect the linearity of power amplifiers, and a loose
tolerance on dielectric constant will make it very difficult to maintain a tight tolerance on the impedance of a
trace width in a microstrip configuration. In addition, due to the chemical nature of some thermoset resin
systems, the dendrite structure of the copper is not able to bond with the thermoset resin. The addition of
additives and fillers exacerbates this disadvantage. Due to this characteristic, the peel strength of some of the
latest high performance thermoset products are extremely low, making it very difficult (hence increasing the
processing cost) to work with, and extremely expensive when rework is warranted. Furthermore, due to the
low peel strength some of the newer products are not offered in ½ oz copper (18 µm thickness) making it
impossible or very difficult to do fine lines and fine features. This feature is not much of a disadvantage for a
RF design, but it is a major disadvantage for the newer high-speed digital applications. Furthermore, since the
processing of these products is different than traditional FR-4 substrates, a premium is charged for processing
these substrates, therefore the cost advantage of processing the “high performance FR-4 like” products is lost.
All these issues have brought the high performance; low cost high frequency substrates industry and its user
back to the drawing board. Is a high performance low cost substrate a reality, or is it still a myth?
After exhaustive analysis, engineers at Taconic Advanced Dielectric Division came to the conclusion that
PTFE based products are still the substrates that give the best electrical and mechanical properties. Building
on all the advantages of PTFE and by incorporating several proprietary technologies, Taconic came up with a
true, low cost RF/microwave substrate; RF-35.
RF-35 is a ceramic filled, low-cost PTFE substrate. RF-35 not only satisfies the price requirement, but also
exceeds every electrical and mechanical property that is sought in a PCB substrate for high frequency
applications. The dielectric constant is 3.5, with a tight tolerance (+/-0.1), offered in thicknesses that are
increments of 10 mil [0.25 mm]. (RF-35 is currently offered in 10, 20, 30, and 60 mil [0.25 mm, 0.5 mm, 0.76
mm & 1.52 mm]). With the introduction of RF-35P in January 2002 the same attributes are now available in
increments of 2mil [0.05 mm]. RF-35P is available in laminate thicknesses of 2 mil, 4 mil, 6 mil & 8 mil [0.05
mm, 0.10 mm, 0.15 mm & 0.20 mm]. The thickness tolerance for RF-35 & RF-35P is per IPC-L-125, Class C
(e.g., the tolerance for a 0.020” [0.5mm] substrate is +/-0.0015”).
RF-35 demonstrates exceptional thickness and dielectric constant variation within a sheet. The standard
deviation of dielectric constant within a sheet is 0.01, and the standard deviation of the thickness within a
sheet is 0.00023”. The loss tangent of RF-35, was measured at n = 1,2,3,… harmonic, and the frequency

anged from 500 MHz to 11.2 GHz. As the figure below indicates, there is virtually no variation in dielectric
constant across the frequency spectrum, and the loss of RF-35 was 0.0018 at 1.9 GHz and 0.0025 at 10 GHz.
Dk Versus Frequency for RF-35
Df Vs. Frequency for RF-35
Normalized Dk
1.04
1.02
1
0.98
0.96
0 2 4 6 8 10 12
Frequency (GHz)
Df
0.003
0.0025
0.002
0.0015
0 1.5 3 4.5 6 7.5 9 10.5 12
Frequency (GHz)
The total loss in db/in versus frequency is also plotted:
0
Frequency Vs. Loss (db/in) for RF-35
(Microstrip Configuration, 30 Mil, 1/2 Oz Copper)
db/in
-0.1
-0.2
0 3 6 9 12
Frequency (GHz)
The water absorption is less than 0.02% when exposed to water for 24 hours at 23 o C. RF-35 is available in
½ oz, 1 oz and 2 oz copper, and the peel strength (after the RF-35 substrate is exposed to a solder float @
550 o F [287 o C] for 10 seconds) is greater than 8 pounds/inch [1.4 N/mm] for ½ oz copper. To address the
issue of the rigidity of the PCB substrate, a proprietary technology was used to utilize the structural integrity of
the woven glass reinforcement. As a result of this work, the hardness of the substrate was increased to 34, as
measured by the Rockwell hardness scale. Furthermore, RF-35 was engineered to have the best dimensional
stability. RF-35 has a dimensional change of 40 PPM in the X-axis (fill direction) and a dimensional change of
-100 ppm in the Y-axis (warp direction) after etching the copper. These numbers, which translate into
excellent registration in the finished substrates, are the best that can be found in any PCB substrate. The
CTE in the Z axis for RF-35 is 64 ppm/ o C, which ensures excellent reliability of the plated through holes. To
ensure the mechanical integrity of RF-35 and to dispel the myth that PTFE substrates exhibits creep, RF-35
was tested for creep by an independent lab, according to BSI-125. The results are given below:

Percent Creep Vs. Temperature for RF-35
% Creep
20
0
-20
-40
-60 0 100 200 300 400 500
Temperature, C
As the graph indicates RF-35 does not exhibit any creep/negligible creep when exposed to a load of 4200 psi,
up to a temperature of 550 o F [287 o C]. This result indicates that RF-35 should not exhibit any creep in a
normal PCB application.
What makes RF-35 so unique? What gives RF-35 ALL the electrical and mechanical properties that RF
designers look for? The answer lies in the unique, proprietary process that Taconic has developed. By
modifying the surface chemistry of the ceramic filler, Taconic has been able to get a true interpenetrating
polymer network of the ceramic filler and PTFE. This network ensures that the ceramic particles are
encapsulated with PTFE, which enables RF-35 to get the synergy effects of both, PTFE and the ceramic filler.
Hence as explained above, RF-35 has the best dimensional stability, low CTE, an excellent mechanical
integrity, a tight tolerance on DK and thickness, very high peel strength, and is the cheapest high performance
laminate available in the marketplace today. Furthermore, due to the inherent nature of PTFE, RF-35 has a
flammability of UL-94 V0.
Furthermore, a comparison of the processing steps for RF-35 and for a typical thermoset resin PCB material
indicates that contrary to popular belief, processing RF-35 does not entail a complex process compared to
processing a thermoset resin based PCB.
PROCESS PROCESS SIGNIFICANCE RF-35 THERMOSETS
Hole Quality -PTH Reliability Good Good
Tool Life -Process Cost Good Poor
Burring -Added Process Good Poor
Hole Treatment -Added Process Fair Good
PTH -PTH Quality Good Good
Image, Etch -Trace/Line Definition Good Good
Copper Plating -PTH Reliability Good Good
Strip, Etch -Trace/Line Definition Good Good
Solder Coating -Ease of soldering Good Good
Depanelizing -Finished edge quality Good Good
Routing Tool and Wear -Tool expense and replacement Fair Fair
Routing Feed Rate -Routing cost Fair Fair
Fixturing -Fixturing time and cost Fair Good
Pick and Place -Ease and part location Fair Good
Reflow Soldering -Warping and part integrity Fair Fair
Mounting -Conformance to housing or pallet Good Fair

Conclusions:
RF-35 does offer the combination of the best performance and best price for a high frequency application,
indicating that a high performance, low cost substrate is indeed a reality. As listed in the article, RF-35 offers
all the electrical advantages of PTFE, all the mechanical advantages of the ceramic filler, negates all the
disadvantages of PTFE substrates and disproves all the “myths” surrounding Teflon ® substrates and
processing Teflon® substrates.